The present invention relates to a directional diffusing film and, more particularly, to a directional diffusing film that can diffuse light in a controlled diffusion mode, a method of manufacturing the directional diffusing film, a surface light source unit and a liquid crystal display.
Generally, a surface light source unit is provided with a light diffusing film. The light diffusing film is disposed on a light-emitting side of the surface light source unit to diffuse illuminating light emitted by a light source of the surface light source unit.
A conventional light diffusing film is formed of a material prepared by dispersing a light diffusing matter such as organic or inorganic beads in a transparent resin base, or is formed by coating a transparent resin base with an ink containing a dispersing matter such as organic or inorganic beads.
FIG. 8 is a sectional view of a liquid crystal display 135 provided with an edge-type surface light source unit 120 as an example of a conventional surface light source unit employing a conventional light diffusing film.
As shown in FIG. 8, the surface light source unit 120 includes, as principal components, two light sources 121, a light guide plate 122, a reflecting film 124, and three light diffusing films 110-1, 110-2 and 110-3 that have the same parameters.
The light guide plate 122 is a surface light emitting means and has a light-emitting surface 122a, a not light-emitting surface opposite the light-emitting surface 122a, and opposite side surfaces. The light sources 121 are disposed beside the opposite side surfaces, respectively. A dot pattern 123 is formed on the not light-emitting surface in order to diffuse light emitted by the light sources 121 toward the light emitting surface 122a. The reflecting film 124 is disposed so as to face the not light-emitting surface of the light guide plate 122 to intercept light rays traveling in undesired directions and to reflect back the same in a predetermined direction.
The diffusing films 110-1, 110-2 and 110-3 disposed on the side of the light emitting surface 122a of the light guide plate 122 diffuse light emitted by the surface light source unit 120 to enhance the uniformity of light. The diffusing films 110-1, 110-2 and 110-3 conceal the dot pattern 123. A single diffusing film is unable to diffuse light satisfactorily and diffuses light irregularly. Usually, three diffusing plate or so are necessary to achieve satisfactory diffusion and to enhance front luminance.
A transparent liquid crystal display unit 133 is disposed on the light emitting side of the surface light source unit 120. The liquid crystal display unit 133 has a lower substrate 132, an upper substrate 131 and a liquid crystal layer 130 sandwiched between the substrates 131 and 132. In the liquid crystal display 135 shown in FIG. 8, the liquid crystal display unit 133 is illuminated from behind by the surface light source unit 120.
The light diffusing films included in the aforesaid conventional liquid crystal display contain beads as a diffusing matter. Therefore, light-diffusing characteristic of the light diffusing films is unsatisfactory if particle sizes of the beads are irregularly distributed or if the beads are dispersed unsatisfactorily in a base material of the light diffusing films or a coating material of the light diffusing films. In addition, it is difficult to maintain the light diffusing films in a satisfactory appearance. Furthermore, productivity (yield) in manufacturing the light diffusing films is low.
In addition, it is possible that the beads fall off edges of the light diffusing films when punching or cutting the light diffusing film into a desired size or when assembling the punched or cut light diffusing films, which can produce dust and foreign matters.
The beads diffuse light in all directions. Consequently, the amount of light can be reduced by internal absorption and some part of light can be reflected toward the light guide plate 122. Thus, the amount of light emitted in the normal (front) direction can be reduced, that is, efficiency of utilization of the light emitted by the light sources 121 may be low, and luminance of the surface light source unit 120 may be also low.
It is an object of the present invention to provide a directional diffusing film that can utilize light at a high efficiency and that can be produced at a high productivity free from producing dust and foreign matters, and a method of manufacturing such a directional diffusing film.
According to the present invention, a directional diffusing film includes a base film, and a plurality of convex directionality-providing elements regularly arranged on at least one surface of the base film; wherein a section of a first directionality-providing element in a first plane including a top of the directionality-providing element has a shape of a substantially triangle, and a section of the directionality-providing element in a second plane including the top of the directionality-providing element and perpendicular to the first plane has a top of a second directionality-providing element and a shape of a continuous wave.
The directional diffusing film having characteristics specified by the present invention can diffuse light in proper directionally diffused light, while maintaining high light-diffusing ability. The directional diffusing film utilizes light highly efficiently as compared with conventional diffusing films.
Preferably, the shape of the continuous wave is a periodic waveform having a period corresponding to each of the directionality-providing elements. For example, the periodic waveform may be a substantially sinusoidal waveform.
The shape of continuous wave may be a periodic waveform having wavy parts corresponding to the directionality-providing elements and straight parts respectively extending between the adjacent wavy parts.
Preferably, the section of the directionality-providing element in the first plane has a shape of a substantially isosceles triangle. Preferably, the substantially triangle has a vertex angle of a range of 80xc2x0 to 100xc2x0. Preferably, the substantially triangle has a vertex which is rounded and/or a predetermined amount of which is cut off.
Preferably, the plurality of convex directionality-providing elements are arranged on a light emitting surface of the base film so that incident light of incident angles in a range of 70xc2x0 to 80xc2x0 leaves the directional diffusing film at outgoing angles including a maximum outgoing angle in a range of 25xc2x0 to 40xc2x0. In the case, light that is difficult to be emitted out in prior art can be efficiently emitted out toward the front direction.
Preferably, the plurality of convex directionality-providing elements are arranged on a light emitting surface of the base film so that a haze value is in a range of 70 to 90 when light falls on a light receiving surface of the base film. Thus, luminous intensity (density) of diffused light in a necessary range can be increased.
According to the present invention, a method of manufacturing a directional diffusing film having the aforesaid characteristics comprises: a preparing step of preparing a cylinder having a molding pattern of a shape complementary to the plurality of convex directionality providing elements; and a molding step of molding the plurality of convex directionality providing elements by introducing a resin into the molding pattern of the cylinder.
According to the present invention, the directional diffusing film having the aforesaid characteristics can be manufactured at a manufacturing cost comparable to that of the conventional diffusing film. In addition, shape-repeatability and productivity are improved.
Preferably, the molding step includes: a step of introducing an ionizing radiation hardening resin into the molding pattern of the cylinder; and a step of setting the ionizing radiation hardening resin by irradiating ionizing radiation.
For example, the preparing step includes a step of forming the molding pattern of the shape complementary to the plurality of convex directionality providing elements by cell-engraving by means of an electronic engraving machine provided with a diamond stylus having a point angle in a range of 80xc2x0 to 100xc2x0. Preferably, the diamond stylus has a tip that is rounded and /or cut by a predetermined amount.
In addition, this invention is a surface light unit comprising: a light source, a surface light emitting means having a light-emitting surface that can emit a light from the light source in a predetermined direction, and a directional diffusing film that includes a base film, and a plurality of convex directionality-providing elements regularly arranged on at least one surface of the base film, wherein a section of a first directionality-providing element in a first plane including a top of the directionality-providing element has a shape of a substantially triangle, and a section of the directionality-providing element in a second plane including the top of the directionality-providing element and perpendicular to the first plane has a top of a second directionality-providing element and a shape of a continuous wave.
Alternately, this invention is a liquid crystal display comprising: a light source, a surface light emitting means having a light-emitting surface that can emit a light from the light source in a predetermined direction, a directional diffusing film that includes a base film, and a plurality of convex directionality-providing elements regularly arranged on at least one surface of the base film, and a transparent liquid-crystal element arranged on a light-emitting side of the directional diffusing film, wherein a section of a first directionality-providing element in a first plane including a top of the directionality-providing element has a shape of a substantially triangle, and a section of the directionality-providing element in a second plane including the top of the directionality-providing element and perpendicular to the first plane has a top of a second directionality-providing element and a shape of a continuous wave.
The surface light source unit and the liquid crystal display are able to enhance luminance efficiently in a practical range of luminance, and the liquid crystal display is also capable of clearly displaying images or the like, even if the number of the directional diffusing films is small. The surface light source unit and the liquid crystal display can be formed in smaller thicknesses and can be sold at lower prices, respectively.